In fact, the penis of the common chimpanzee is no shorter than in humans, averaging The average penile girth was about 4. Penis size has been the subject of dozens of studies, as well as countless urban legends, myths and spam emails. And in terms of girth, the average erect circumference was 4. At the time of their measurement, some men were with partners or "a friend," although most employed a DIY method for attaining an erection.